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A planar lens composed of a chirped subwavelength grating (CSG) structure with high numerical aperture (NA) was designed and analyzed in this paper. The reflectivity, transmission, and phase were calibrated as a function of the grating dimension using rigorous coupled wave analysis, while the focusing properties were numerically simulated by finite-element method. The designed CSG focused the reflected and transmitted waves that have approximately the same power ratios simultaneously. Numerical aperture values of the planar lens as high as 0.91 and 0.92 were obtained for normal incidence of TM and TE polarization light, respectively.